Is it something I (no idea on CHDK port process) could help/start or some more experienced CHDK dev is needed?

The process itself is simple: you make a copy of a port that is sufficiently close to your cam, then you adapt it to work on your camera.Copy loader/g16 to loader/s120, platform/g16 to platform/s120. Inside platform/s120/sub, you keep one of the directories and rename it to match your firmware version. Put the firmware dump you got from the dumper script in platform/s120/sub/{your fw version}.Now, you'll need to go through all files in the s120 directories and adapt their contents to your firmware. For assembly parts, you need to disassemble matching parts of your firmware and then make changes to them like it was done in the original port. You'll also need to get all function / variable addresses that are in stubs_entry_2.S and stubs_min.S files. To do that, you'll probably need a copy of one or more "reference" firmwares from here and disassemble them along with your firmware.

Before you start, you need to set up a toolchain and be able to compile CHDK (any of the existing ports). If you're on Windows, you can use this GUI, but you might need to use the command line if the GUI doesn't support some of the operations. Source is here.

The process itself is simple: you make a copy of a port that is sufficiently close to your cam, then you adapt it to work on your camera.Copy loader/g16 to loader/s120, platform/g16 to platform/s120. Inside platform/s120/sub, you keep one of the directories and rename it to match your firmware version. Put the firmware dump you got from the dumper script in platform/s120/sub/{your fw version}.Now, you'll need to go through all files in the s120 directories and adapt their contents to your firmware. For assembly parts, you need to disassemble matching parts of your firmware and then make changes to them like it was done in the original port. You'll also need to get all function / variable addresses that are in stubs_entry_2.S and stubs_min.S files. To do that, you'll probably need a copy of one or more "reference" firmwares from here and disassemble them along with your firmware.

As the risk of shedding light where there is no darkness, what helps a lot is a reference port having comments in its code that tell you a bit about how the various #define values & routines were found. Sometimes there might be a note about how it matches a different port and sometimes a note about how it's different. The G16 port (and many others) was pretty much done that way so it's probably not a bad reference for an S120 port.

So .. as you do the S120 port, adding detailed comments about what you've found to help the next person will be most appreciated. At a minimum, please removed unrelated / unverified comments that show up as you c&p source.